Python returns MagicMock object instead of return_value

Question

I have a python file a.py which contains two classes A and B.

class A(object):
    def method_a(self):
        return "Class A method a"

class B(object):
    def method_b(self):
        a = A()
        print a.method_a()

I would like to unittest method_b in class B by mocking A. Here is the content of the file testa.py for this purpose:

import unittest
import mock
import a


class TestB(unittest.TestCase):

    @mock.patch('a.A')
    def test_method_b(self, mock_a):
        mock_a.method_a.return_value = 'Mocked A'
        b = a.B()
        b.method_b()


if __name__ == '__main__':
    unittest.main()

I expect to get Mocked A in the output. But what I get is:

<MagicMock name='A().method_a()' id='4326621392'>

Where am I doing wrong?

When testing, A() returns the return_value from mock_A (a regular MagicMock, as you haven't specified anything else), which is not an instance of the class A. You need to set that return_value to be something that has a defined method_a. — jonrsharpe
mock_a.method_a.return_value = 'Mocked A' => mock_a().method_a.return_value = 'Mocked A' should be better :) — Ali SAID OMAR
@AliSAIDOMAR is precisely correct, it's the return value from calling mock_a that should have the method, not mock_a itself. — jonrsharpe
@jonrsharpe . Thanks for your explanation. I just tried. Both mock_a().method_a.return_value = 'Mocked A' and mock_a.return_value.method_a.return_value = 'Mocked A' worked. Thanks a lot for your comments. Would you please go ahead and put it as an answer? — Mehdi Jafarnia Jahromi

jonrsharpe jonrsharpe · Accepted Answer · 2016-07-05T09:05:29

When you @mock.patch('a.A'), you are replacing the class A in the code under test with mock_a.

In B.method_b you then set a = A(), which is now a = mock_a() - i.e. a is the return_value of mock_a. As you haven't specified this value, it's a regular MagicMock; this isn't configured either, so you get the default response (yet another MagicMock) when calling methods on it.

Instead, you want to configure the return_value of mock_a to have the appropriate method, which you can do as either:

mock_a().method_a.return_value = 'Mocked A' 
    # ^ note parentheses

or, perhaps more explicitly:

mock_a.return_value.method_a.return_value = 'Mocked A'

Your code would have worked in the case a = A (assigning the class, not creating an instance), as then a.method_a() would have triggered your mock method.

Python returns MagicMock object instead of return_value

1 Answers